In a conventional magnetic data storage system it is known to store digital data from a host device, eg a computer. It is known to store digital data to a magnetic disk or tape by switching the polarity of current through a magnetic write head which is in close proximity to a magnetic media. Conventionally, the magnetic media may comprise a flexible elongate tape which is coated with a magnetic material and which is wound between two reels past a magnetic write head. Alternatively, the magnetic media may also comprise a rigid disk which is coated with a magnetic medium and data is recorded to the disk by moving a recording head in a radial direction across the disk while the disk is rotated about its center.
In tape-based magnetic data recording systems, data may be recorded using a plurality of write heads and is read with a plurality of read heads. Conventionally, these write and read heads may be either substantially stationary with respect to the rest of the device in which case data are stored in a plurality of tracks parallel to the elongate direction of the tape or the write and read heads may be mounted on a drum which is rotated about an axis at an angle to the elongate direction of the tape, in which case the data are stored in a series of tracks diagonally across the magnetic tape.
Conventionally, recording heads are fabricated from ferrite which is a sintered combination of a ferro magnetic material and a ceramic combined to yield a material with the high magnetic permeability of the former and the high electrical resistance of the latter. However, writing data to magnetic media using ferrite heads becomes more inefficient at high data bit-rates. At high frequencies the losses due to irreversible heating of the write head results in a roll-off of the output of the write head for a given input current.
Referring to FIG. 1 herein, there is illustrated by a solid line 110 a plot indicating how an output signal of a ferrite write head decreases as the frequency of a driving write current signal of constant amplitude increases. The output signal for a given input current drops significantly at a "roll off" frequency 120 which is, typically of the order 30-40 MHz. This limits an effective maximum write rate of the ferrite head to the order 80 MBits/s. It is known to attempt to correct for this roll off in the frequency response of a write head by preferentially boosting high frequencies in the write current according to a curve such as illustrated by a dashed line 130 in FIG. 1. Preferentially boosting high frequencies in the write current signal driving the write head to compensate for the decrease in efficiency of the write head should yield an approximately flat frequency response as illustrated by the dot dashed line 140 in FIG. 1. This technique of boosting the high frequencies is conventionally known as "Write Pre-Equalization" (WPE). However, write drivers in digital magnetic data stored systems are highly non-linear devices. Conventional write drivers comprise switches which send two polarities of current to the write heads in order to record two distinct magnetization states on the magnetic media. Hence, any prior art attempts to boost high frequencies in such devices have been complex. In particular, Ampex produced a write pre-equalization scheme which comprised a linear amplifier current driver i.e. the output signal of the write driver was proportional to the input signal to the write driver. Having produced a linear write driver, the Ampex scheme applied a boost to the frequency response of the write driver to compensate for the frequency response to the write head. However, the Ampex implementation of write pre-equalization required substantial power, typically of the order 15 Watts, and could only be produced using discrete components. Hence it is not possible to implement this as a single application specific integrated circuit (ASIC). In addition, the Ampex scheme was also difficult to set up.
In addition to the roll-off in the frequency response of a recording head at high frequencies as described hereinbefore there is another, more significant, effect resulting from a finite rise time of a magnetic field generated by a record head in response to a substantially step-like change in a recording current driving said record head. In response to a, for example, positive going edge of a driver current the resulting magnetic field starts to increase in magnitude with what we shall call a positive sign. In response to a following negative going edge in the recording head driver current the magnetic field will first reduce in magnitude from a large positive value, will become zero and will then increase in magnitude with a negative sign. The changes in magnetic field do not occur instantaneously, but do so with a finite risetime, for example, 7 nanoseconds (ns).
The magnetization of the medium changes direction during the time that the magnetic field is increasing in magnitude from zero. If, at the time of the negative going edge in the recording head driver current, the magnetic field has not yet risen to its steady-state level then the next magnetic transition occurs slightly earlier than if the magnetic field had been at (or closer to) the steady-state level. This causes the physical location on a magnetic data storage medium at which the magnetization of the medium changes direction to be "advanced", or moved closer to the previous transition. Hence, positions of magnetic field transitions on a magnetic data storage medium may be laterally displaced. This lateral displacement of magnetization on the magnetic recording medium is also known herein as "bit shift", "peak shift" and "transition shift".
The size of the transition shift depends in a non-linear way on the magnetic field level when last current transition occurred, and hence on the duration of the previous current pulse relative to the finite risetime of the record head's magnetic field.
The effect of these non-linear lateral displacements of regions of magnetization on magnetic recording media can result in timing errors during a subsequent read operation of data stored on the storage medium and may result in an increase in the number of errors occurring during the read operation.
The ongoing pressure in the development of new magnetic data storage systems is to increase the data storage capacity of any said data storage media. By increasing the effective bandwidth of the write head in a magnetic data storage system it is possible to increase the bit rate at which data is written to, for example, magnetic tapes and hence increase the storage capacity of the tape. There is a need for a means to increase the bandwidth of magnetic recording heads in a way which can be implemented as an ASIC and which is straightforward to both calibrate and use.
In the applicant's co-pening application "Double Pulse Write Driver" Ser. No. 09/313,453, filed concurrently with this application, the full contents of which are incorporated herein by reference, there is disclosed a method and apparatus for compensating for the roll-off and peak shift in magnetic flux of a record head. In such a method and apparatus, there exists a problem of optimizing the compensation means to minimise the number of errors occurring whilst reading data during a send operation.